Errors during meiotic cell division are a leading cause of mental retardation and pregnancy loss in our species. The vast majority of human meiotic errors are maternal in origin and the rate of errors is strongly influenced by age. While the mechanism(s) of error and the way in which age influences chromosome segregation remain unknown, recent studies have demonstrated that the number and placement of meiotic recombination events profoundly influences chromosome segregation. In the past several years our understanding of the molecular events involved in meiotic recombination has increased dramatically, yet we remain largely ignorant of the factors that control recombination levels and placement in mammals. The studies outlined in this application are predicated on the assumption that recombination is influenced by the interplay of prophase events involved in the establishment of cohesion between sister chromatids, the formation of the synaptonemal complex between homologs, and the repair of DNA double stand breaks. Accordingly, we propose three sets of interrelated studies, one to define the key events in prophase of mammalian female meiosis and to assess the impact of "normal"genetic variation on these processes;one using mutational analysis to examine the effect of "abnormal" variation on these events;and one to examine the downstream effects of perturbations in prophase. This approach, using the meiotic "reagents" developed during the initial funding period, will allow us to test specific hypotheses about the role of cohesion, DMA sequence, and the synaptonemal complex in the establishment of meiotic exchanges. The combined data from these studies will allow us to understand the control of recombination In mammals and how the events of prophase influence meiotic chromosome segregation.